The Way We Define A Kilogram Could Change Next Week

Ryan F. Mandelbaum

A long-discussed resolution for next week’s General Conference on Weights and Measures would toss out the international prototype of the kilogram — a hunk of platinum and iridium in Paris that everyone agrees weighs 1 kilogram. Instead, the International Bureau of Weights and Measures (BIPM) would redefine the International System of Units to ensure that kilograms are based on things that can’t change over time. It would require an entire rethinking of how kilograms work.

As we’ve previously reported, there’s a reason scientists rely on a hunk of metal. King Louis XVI created a standardised weight to stop tradespeople from scamming customers, which was called the grave and was equal to a litre of water held at zero degrees Celsius. In 1795, French leaders decided instead to go with the gram, a cubic centimeter of water held at 4 degrees Celsius. The hunk of metalreplaced the inconvenient water measurement in 1799, and 17 countriessigned a treaty in 1875 to standardize their mass to the international prototype of the kilogram, or le grand k. Today, over a hundred countries, including the U.S., have signed an agreement to use the BIPM’s services.

But even today, it’s all based on comparison. Le grand k remains secured under lock and key (three keys, actually) in Paris. Countries can get copies of le grand k, and then must use scales calibrated to the copy of the real thing. But it’s possible that something can get lost in the bucket brigade — and of course, there’s a chance the original goes missing or gets destroyed. Comparisons with copies have also revealed that the mass of the kilogram (or the mass of the copies) has slightly changed, albeit an incredibly tiny amount. It’s possible the copies absorbed molecules from the air, or the original lost mass through washing or other physical processes, NPR once reported.

The proposal to be voted on next week would drop the kilogram as the last unit still based on a physical thing on the Earth. Instead, we would use the universe itself as a measure. Built into the fabric of spacetime are fundamental constants that scientists can measure in experiments. The speed of light in a vacuum is always the same velocity, 299,792,458 meters per second. The ratio of a light particle’s energy to its frequency is equal to 6.62607015×10–34 Joule-seconds, where a joule’s units are kilograms times meters squared over seconds squared; this number is called Planck’s constant. Meters and seconds have already been redefined by the BIPM to align with the speed of light and the vibration of a cesium atom, respectively. A kilogram would now be defined based on the fixed value of Planck’s constant.

“With the redefinition, we can measure mass out of thin air using this constant of the universe,” Stephan Schlamminger, physicist at the National Institute of Standards and Technology, told Gizmodo. “I think that’s a substantial improvement.”

In practice, those hoping to determine the amount of mass equal to a kilogram would use a device called a Kibble balance, which measures an amount of electromagnetic force required to balance a test mass.

The vote is scheduled for November 16, and the resolution is expected to go through, reports the Guardian. So if you really want to understand mass, it might just be time to learn a little quantum mechanics.